Neuroinflammatory Dynamics Underlie Memory Impairments after Repeated Social Defeat

Impaired working memory after RSD. A, Representative time line of working memory assessment in the MWM. Mice were subjected to six cycles of RSD or left undisturbed as CONs. Working memory was examined for 5 d (D7–D11) after completion of RSD (n = 10). B, In this paradigm, the escape platform was moved to a different quadrant each testing day. C, Representative paths for CON and RSD mice in the MWM on day 3. Velocity (D), latency to the platform (E), distance traveled (F), and percentage time in the outer annulus (G) were determined for all 5 testing days. Graphs represent the mean ± SEM. *p < 0.05, means are significantly different from CONs; ^#p = 0.06–0.10, means tended to be different from CONs.

Impaired spatial memory recall in the MWM after RSD. A, Representative time line of learning and memory recall assessments in the MWM. Mice were subjected to six cycles of RSD or left undisturbed as CONs. Learning was examined after the completion of RSD for 5 d (D7–D11), and then memory recall was tested (D12; n = 8). B, In this paradigm, the escape platform remained in the same location each testing day of the acquisition phase but was removed during the probe trial. During the acquisition phase, latency to the platform (C) and distance traveled (D) were determined each of the 5 d of testing. E, Representative search paths of CON and RSD mice during the probe trial (D12). Spatial memory recall was assessed during the probe trial, and distance traveled (F) and percentage of time in the target quadrant (G) were determined. Graphs and bars represent the mean ± SEM. *p < 0.05, means are significantly different from CONs.

Impaired spatial memory recall in the Barnes maze after RSD. A, Representative time line of cognitive assessment in the Barnes maze after RSD. Mice were trained on the Barnes maze to acquire learning before social defeat (Pre-Con and Pre-RSD). Next, mice were subjected to six cycles of RSD or left undisturbed as CONs, and memory recall was determined on D7 and D8. Escape latency (B) and total errors (C) are shown before and after RSD exposure. In a related experiment, mice were used as above, and memory recall was determined at 34 and 35 d. Escape latency (D) and total errors (E) are shown before and after RSD exposure. Lines represent the mean ± SEM. *p < 0.05, means are significantly different from CONs; ^#p = 0.06–0.10, means tended to be different from CONs.

Number of proliferating progenitor cells and young neurons in the hippocampus remain unchanged 0.5 d after RSD. Mice were subjected to six cycles of RSD or left undisturbed as CONs. BrdU was injected during the last three cycles of social defeat. BrdU⁺ and DCX⁺ cells were determined in the DG 0.5 d later (n = 6). A, Representative images of BrdU labeling in the DG for CON and RSD mice. B, Quantification of BrdU labeling in the DG for the entire volume of the hippocampus. C, Representative images of DCX⁺ neurons in the DG for CON and RSD mice. D, Quantification of DCX labeling in the DG for the entire volume of the hippocampus. E, Representative images of mature and immature DCX⁺ neurons. F, Quantification of immature DCX labeling and mature DCX labeling. Bars represent the mean ± SEM.

Reduced proliferating neurons in the hippocampus 10 d after RSD. Mice were subjected to six cycles of RSD or left undisturbed as CONs. BrdU was injected during the last three cycles of social defeat. BrdU⁺ and DCX⁺ cells were determined in the DG 10 d later (n = 6). A, Representative images of BrdU/DCX double labeling in the DG are shown. The insets represent a BrdU⁺/DCX⁺ cell (yellow) and BrdU⁺/DCX⁻ cell (green). B, Percentage of BrdU⁺ cells that were also DCX⁺ in the DG. Bars represent the mean ± SEM. *p < 0.05. means are significantly different from CON.

Impaired development of hippocampal NPCs into mature neurons 28 d after RSD. Mice were subjected to six cycles of RSD or left undisturbed as CONs. BrdU was injected during the last three cycles of social defeat. BrdU⁺, NeuN⁺, and GFAP⁺ cells were determined 28 d later (n = 6). A, Representative images of BrdU labeling are shown in the DG. B, Quantification of BrdU⁺ cells per section of the hippocampus. C, Representative images of NeuN/BrdU labeling are shown in the DG. The arrows show the cell depicted in the inset. D, Quantification of NeuN⁺/BrdU⁺ cells per section of the hippocampus with rostral–caudal distinction. E, Representative images of GFAP/BrdU labeling are shown in the DG. The arrows show the cell depicted in the inset. F, Quantification of GFAP⁺/BrdU⁺ cells per section of the hippocampus. Bars represent the mean ± SEM. *p < 0.05, means are significantly different from CON.

Social avoidance behavior was present 28 d after RSD. Mice were subjected to six cycles of RSD or left undisturbed as CONs, and social interaction was determined 28 d later (n = 14). A, Representative time line of the experiment is shown. B, Interaction paths are shown for the Empty (Trial 1, top) and Social (Trial 2, bottom) trials. C, Time spent in the interaction zone during the Empty and Social Trials. D, Time spent in the corner zone during the Empty and Social Trials. Bars represent the mean ± SEM. *p < 0.05, means are significantly different from all other groups.

Minocycline intervention attenuated RSD-induced microglia activation and monocyte recruitment. Mice were subjected to six cycles of RSD or left undisturbed as CONs. Minocycline (90 mg/kg) was administered in drinking water throughout the 6 d of RSD. A, Representative images of Iba-1 labeling in the DG are shown. B, Proportional area quantification of Iba-1. C, Representative images of CD45 labeling in the DG are shown. B, Quantification of CD45⁺ cells. E, A coronal brain section was collected 14 h after the last cycle of stress, and the mRNA level of IL-1β was determined. Bars represent mean ± SEM. *p < 0.05, means are significantly different from CON.

Minocycline prevented spatial memory impairments after RSD. Mice were trained on the Barnes maze to acquire learning before social defeat (Pre-Veh CON, Pre-Veh RSD, and Pre-Mino RSD). The Pre-Mino RSD group received minocycline (90 mg/kg) in drinking water 1 d before RSD and continued throughout the RSD period. Mice were subjected to six cycles of social defeat (Veh RSD and Mino RSD) or left undisturbed as CONs (Veh CON), and memory recall was determined on D7 and D8. A, Representative time line of cognitive assessment in the Barnes maze after RSD (n = 6–9). Escape latency (B) and errors (C) are shown before and after RSD exposure. *p < 0.05, means are significantly different from CON mice; #p = 0.06, mean tended to be different from CONs.